Vacuum system pipe couplings

ABSTRACT

A vacuum system pipe coupling includes a first coupling member and a second coupling member. The vacuum system pipe coupling has a seal system that is to be positioned to seal between the first and second coupling members and a securing system to releasably secure the first coupling member to the second coupling member. The seal system has an inner seal element and an outer seal element spaced from the inner seal element. The first coupling member is provided with a flow passage configured to conduct a pressurised gas to a space between the inner and outer seal elements to shield the outer seal element from fluid flowing through the pipe coupling in the event of failure of the inner seal element.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Section 371 National Stage Application ofInternational Application No. PCT/GB2014/053054, filed Oct. 10, 2014,which is incorporated by reference in its entirety and published as WO2015/055985 A1 on Apr. 23, 2015 and which claims priority of BritishApplication No. 1318127.6, filed Oct. 14, 2013.

FIELD OF THE INVENTION

The invention relates to vacuum system pipe couplings.

BACKGROUND

A vacuum system pipe coupling may be used to couple piping in an exhaustsystem of a vacuum system. The coupling may comprise a first couplingmember having a flange, a second coupling member having flange, anO-ring disposed between the flanges to seal between them and a clamp toreleasably clamp the flanges together. The clamp applies an axial forceto the flanges to compress the O-ring.

Vacuum pumps are frequently deployed in applications that involvepumping substantial quantities of corrosive fluids, including halogengases and solvents. Such materials attack the O-rings of pipe couplings,with the result that the O-ring may become excessively plastic or verybrittle. This can badly affect the integrity of the seal provided.

The intensity of the attack on the O-ring is dependant on a number ofvariables including, for example, the pumped fluid, the material fromwhich the O-ring is made and the pump temperature. In exhaust systems, afurther variable may be the impact of trace heating used to preventcondensation forming in the exhaust piping.

These problems are particularly acute when pumping reactive gases, suchas fluorine, from semi-conductor processing equipment, where gascompositions are varied by reactions in the equipment. Here, even aprecise knowledge of the gas flows admitted to the process chamber is avery poor predictor of the quantity or nature of the reactive gasadmitted to the pump and hence expelled through the exhaust system.

The discussion above is merely provided for general backgroundinformation and is not intended to be used as an aid in determining thescope of the claimed subject matter. The claimed subject matter is notlimited to implementations that solve any or all disadvantages noted inthe background.

SUMMARY

The invention provides a vacuum system pipe coupling as specified inclaim 1.

The invention also includes a vacuum system as specified in claim 15.

The invention also includes a method of protecting a seal system of avacuum system pipe coupling as specified in claim 29.

The invention also includes a vacuum system pipe coupling as specifiedin claim 33.

The Summary is provided to introduce a selection of concepts in asimplified form that are further described in the Detail Description.This summary is not intended to identify key features or essentialfeatures of the claimed subject matter, nor is it intended to be used asan aid in determining the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the disclosure that follows, reference will be made to the drawingsin which:

FIG. 1 is a schematic illustration of a vacuum system including a vacuumsystem pipe coupling;

FIG. 2 is a longitudinal section view through the vacuum system pipecoupling of FIG. 1;

FIG. 3 is an exploded view of the upper half of the vacuum system pipecoupling shown in FIG. 2;

FIG. 4 is a section view of a seal system of the vacuum system pipecoupling;

FIG. 5 is a schematic illustration of a pipe system comprising thevacuum system pipe coupling of FIGS. 2 to 4; and

FIG. 6 is a section view of another seal system of a vacuum system pipecoupling.

DETAILED DESCRIPTION

Referring to FIG. 1, a vacuum system 10 comprises a processing chamber12, piping 14 connecting the processing chamber with a vacuum pump 16(the vacuum pump may be directly connected to the processing chamber andthe piping omitted), an exhaust system 18 leading from the vacuum pumpand comprising a vacuum system pipe coupling 20 connecting the vacuumpump with exhaust piping 22. The exhaust piping 22 may exhaust directlyto atmosphere, optionally via filters or traps. Alternatively, theexhaust piping may conduct the exhaust from the vacuum pump to furtherprocessing equipment. Filters, traps and the processing of the exhaustfrom vacuum pumps are all known to those skilled in the art and will notbe described further herein.

Referring to FIGS. 2 and 3, the vacuum system pipe coupling 20 comprisesa first coupling member 24 having a first flange 26, a second couplingmember 28 having a second flange 30, a seal system 32 disposed betweenthe two flanges to seal between them and a securing system 34 (FIG. 3)to releasably secure the first flange to the second flange. The sealsystem 32 comprises an inner seal element 36 and an outer seal element38 that is spaced from the inner seal element. The first coupling member24 is provided with a flow passage 40 to conduct a pressurised gas to aspace 42 between the inner and outer seal elements 36, 38. Thepressurised gas can shield the outer seal element 38 in the event theinner seal element 36 fails.

The first coupling member 24 has a longitudinal axis 44 (FIG. 2) andcomprises an annular centre section 46 connected on one side with thefirst flange 26 and on the opposite side with a pipe stub 48. The firstflange 26 comprises a planar sealing face 50 that defines one end of thefirst coupling member 24 and an inclined clamping face 52 disposedgenerally opposite the sealing face. The inclined clamping face 52extends outwardly from the centre section 46 and is inclined towards thesealing face 50 such that in a radially outward direction, the firstflange 26 narrows.

The flow passage 40 comprises a first bore 60 that extends perpendicularto the longitudinal axis 44 and a second bore 62 that extends from thefirst bore to the sealing face 50. The second bore 62 is inclined withrespect to the longitudinal axis 44 in the same direction as theclamping face 52, but in the illustrated example is inclined at adifferent angle. The first bore 60 may be configured to allow connectionwith standard pneumatic fittings and may, for example, be threaded forconnection with an M5 fitting.

The second coupling member 28 has a longitudinal axis 68 (FIG. 2) thatis coaxial with the longitudinal axis 44 of the first coupling member 24such that the axes 44, 68 define a longitudinal axis of the vacuumsystem pipe coupling 20. The second coupling member comprises a centresection 70 connected on one side with the second flange 30 and on theopposite side with a pipe stub 72. The second flange 30 comprises aplanar sealing face 74 that defines one end of the second couplingmember 28 and an inclined clamping face 76 disposed generally oppositethe sealing face. The inclined clamping face 76 extends outwardly fromthe centre section 70 and is inclined towards the sealing face 74 suchthat in the radially outward direction, the second flange 30 narrows.The two clamping faces 52, 76 are inclined in opposite directions sothat the portions of the two flanges 26, 30 disposed radially outwardlyof the respective centre sections 46, 70 present a generallyfrusto-conical cross-section.

The second coupling member 28 comprises a flow passage 80 to receivepressurised gas from the space 42. The flow passage 80 comprises a firstbore 82 that extends perpendicular to the longitudinal axis 68 and asecond bore 84 that extends from the first bore to the sealing face 74.The second bore 84 is inclined with respect to the longitudinal axis 68in the same direction as the clamping face 76, but in the illustratedexample is inclined at a different angle. The first bore 82 may beconfigured to allow connection with standard pneumatic fittings and may,for example, be threaded for connection with an M5 fitting.

The first and second coupling members 24, 28 may be made of any suitablemetal or engineering plastics material and may be metal castings. Asbest seen in FIG. 2, the first and second coupling members 24, 28 mayhave different diameter pipe stubs 48, 72. The first coupling member 24is shown in a configuration in which the flange and pipe stub outsidediameters correspond in proportion to those of a standard KF or NWfitting. To accommodate the flow passage 40, the inside diameter of thefirst flange 26 and centre section 46 is reduced as compared with astandard fitting and there is a tapered transition section 49 betweenthe inner surface of the centre section and the inner surface of thepipe stub 48. The second coupling member 28 accommodates the flowpassage 80 by having a second flange 30 and centre section 70 that whencompared with the proportions of a standard KF or NW fitting areoversize in relation to the pipe stub 72. For example, the pipe stub 72may have proportions corresponding to a standard NW40 fitting, while thesecond flange 30 and centre section 70 have inner diametersproportionate to an NW40 fitting and outer diameters proportionate to anNW50 fitting. It is to be understood that the illustrated configurationis not essential and that the second coupling member 28 may beconfigured to have proportions corresponding to the first couplingmember 24 and vice versa.

As best seen in FIG. 4, the inner and outer seal elements 36, 38 areconnected by an integral web 90 to form a unitary body. The web 90 actsas a spacer determining the radial positioning of the outer seal element38 relative to the inner seal element 36. The inner and outer sealelements 36, 38 may each have a circular cross section and may havesubstantially the same diameter. The web 90 has a width W in a directionparallel to the longitudinal axis of the vacuum system pipe coupling 20that is less than the diameter (width) of the inner seal element 36 andless than the diameter (width) of the outer seal element 38 so that whenthe seal elements are disposed between and in engagement the first andsecond flanges 26, 30, the space 42 is defined between the opposedsealing faces 50, 74 and the inner and outer seal elements 36, 38. Incross-section, the web 90 has a longitudinal axis disposed perpendicularto and coplanar with the respective longitudinal (circumferentiallyextending) axes 92, 94 of the inner and outer seal elements 36, 38 sothat the web is disposed substantially centrally with respect to theseal elements. Accordingly, when the sealing system 32 is installed inthe vacuum system pipe coupling 20, the web 90 is disposed generallycentrally in the space 42. The web is provided with at least onetransverse through-hole 96 to permit pressurised gas supplied from theflow passage 40 to fill the space 42 on either side of the web.

The inner and outer seal elements 36, 38 comprise materials that aredifferent. The outer seal element 38 is made from a first materialselected from materials having relatively good mechanical properties,for example resilience and resistance to compression set. The firstmaterial may, for example, be a fluoroelastomer such as Viton® made andsold by DuPont. The second material is selected from materials havingrelatively good resistance to attack by aggressive chemicals that mayflow through the vacuum system pipe coupling 20 or relatively goodstability at high temperatures. The second material may be aperflouroelastomer such as a grade of Kalrez® made and sold by DuPont ora grade of Perlast® made and sold by Precision Polymer Engineering Ltd.Kalrez® and Perlast® have both been found to provide reliable, long-termservice with a wide range of aggressive industrial and electronic gradechemicals of the type that may flow through the vacuum system pipecoupling 20, particularly when used in highly aggressive chemicalprocessing and semiconductor wafer processing applications.Perflouroelastomers also have relatively good high temperaturestability. Grades of Kalrez® and Perlast® are rated for maximumcontinuous service temperatures in excess of 275° C. and up to 327° C.

The materials from which the first and second seal elements are made mayhave a Shore hardness of 60 to 80, with 70 being a currently preferredvalue.

In the example illustrated in FIG. 4, the outer seal element 38, web 90and a core, or substrate, 102 of the inner seal element 36 are made ofthe same first material and the inner seal element further comprises asleeve 104 made of the second material. The outer seal element 38, web90 and core 102 may be made of a selected grade of Viton® or anothermaterial selected for having relatively better resilience or resistanceto compression set than the sleeve 104. The sleeve 104 may be made of aselected grade of Kalrez® or Perlast® or another material selected forhaving relatively better resistance to chemical erosion or hightemperature stability than the material from which the outer sealelement 38, web 90 and core 102 are formed. Since polymers such asKalrez® and Perlast® are very expensive, having a core made of arelatively cheap material, such as Viton®, sleeved with the moreexpensive material reduces the cost of the seal system 32. The sealingelements 36, 38 and web 90 may be made by a co-moulding process.

The seal system 32 further comprises an inner seal carrier 106 and anouter seal carrier 108. The inner seal carrier 106 is an annular bodymade of a suitable metal, or engineering plastics material, and isdisposed radially inwardly of the inner seal element 36. The inner sealcarrier 106 has a generally rectangular cross-section with a slightlyconcave face facing the inner seal element 36 in order to facilitatemating of the two parts. The outer seal carrier 108 is an annular bodymade of a suitable metal or engineering plastics material. The outerseal carrier 108 has a generally T-shaped section, or profile, definedby an inner body portion having a cross-section similar to, or the sameas, the inner seal carrier 106 and a wider plate-like outer portiondisposed at the end of the body portion opposite the end provided withthe concave face. The outer seal carrier 108 provides support for theouter seal element 38 and centres the coupling members 24, 28. In otherexamples, the outer seal carrier may be shaped generally as the innerseal carrier shown in FIGS. 2 and 3 and the inner seal carrier shapedgenerally as the outer seal carrier shown in those drawings so that itis the inner seal carrier that centres the coupling members 24, 28.

Referring to FIG. 3, the securing system 34 comprises a clamp that isused to releasably secure the first and second coupling members 24, 28to one another by engaging the clamping faces 52, 76 and pressing thecoupling members towards one another. The clamp may take any suitableform and may, for example, be one such as those known in the art forclamping KF or NW fittings. Since such clamps will be known to thoseskilled in the art, they will not be described in detail herein.

As shown in FIG. 5, in use the vacuum pipe system coupling 20 may befitted into piping, such as the piping of the exhaust system 18 shown inFIG. 1, by connecting the pipe stubs 48, 72 to respective lengths ofpiping. The pipe stub 48 may be connected to a length of piping 112 andthe pipe stub 72 connected to a length of piping 114. The respectiveconnections between the pipe stubs 48, 72 and the piping 112, 114 may bemade by any known method that is convenient, for example bycircumferential welds 115 formed using an orbital welding process. Theseal system 32 is disposed between the sealing faces 50, 74 of the firstand second coupling members 24, 28, which are secured to one another bythe securing system 34. The securing system 34 acts on the clampingsurfaces 52, 76 to press the first and second coupling members 24, 28together so that the inner and outer seal elements 36, 38 are compressedbetween the sealing faces 50, 74 to seal the connection. As shown inFIG. 2, the compression of the inner and outer seal elements 36, 38results in a flattening of the sides of the seal elements against thesealing faces 50, 74.

The first flow passage 40 is connected with a gas module 116 via piping118 and a pneumatic fitting 120 provided on an end of the piping andfitted into the upstream end of the first bore 60. Pressurised gas issupplied from the gas module 116 to the first flow passage 40 whichconducts the pressurised gas to the space 42 defined between the sealingfaces 50, 74 and the inner and outer seal elements 36, 38. A one-wayvalve 122 and a flow restrictor 124 are connected with the piping 118upstream of the first flow passage 40. A first transducer 126 isconnected with the piping 118 so as to be able to sense, or detect, thepressure of the pressurised gas in the piping upstream of the flowrestrictor 124. The one-way valve 122 prevents contamination of gas atthe gas module 116 by backflow that may occur in the event the pressurein the space 42 rises above the pressure at which the pressurised gas issupplied by the gas module 116. The flow restrictor 124 may be made froma slightly porous material that inhibits the flow of gas such that itacts like a dam, allowing a trickle of gas to pass through.Alternatively, the flow restrictor 124 may be a fine metering valve or afine capillary hole provided in a solid material.

The downstream end of the second flow passage 80 is connected with apneumatic fitting 128 provided on an end of piping 130. The piping 130receives pressurised gas that has passed from the first flow passage 40through the space 42 and into the second flow passage 80 to conduct thegas away from the vacuum system pipe coupling 20. The piping 130 mayreturn the pressurised gas via suitable filters, traps or other suitableequipment 136 for resupply by the gas module 116 or conduct it to thefirst flow passage 40 of another vacuum system pipe coupling 20. Asecond transducer 132 is connected with the piping 130 to sense thepressure of the pressurised gas that has exited the second flow passage80. The first and second transducers 126, 132 are connected with acontroller, or determining unit, 134 to pass signals indicative of thesensed pressures P1 (transducer 126) and P2 (transducer 132) to thecontroller.

The supply of pressurised gas is controlled by the gas module 116. Thegas module 116 may comprise an active manifold that regulates a supplyof gas from a reservoir 138. The gas module 116 is set to supplypressurised gas into the piping 118 at a pressure above that of theexhaust gases flowing through the vacuum system pipe coupling 20. Thesupply pressure may be 2 Bar (approximately 200 KN/m²). The gas module116 is configured to send signals to the controller 134 indicating thecharacteristics of the gas supplied into the piping 118, for example theflow rate and gas pressure. In addition to supplying the pressurised gasto the vacuum system pipe coupling 20, the gas module 116 may be used todistribute a purge gas to different locations within a vacuum system ofwhich the pipe coupling is apart.

In use, pressurised gas from the gas module 116 is supplied to the space42 via the piping 118 and first flow passage 40. The pressurised gasserves to inflate the sealing system 32 and passes from the space 42 tothe second flow passage 80 and on into the piping 130. In normalconditions in which the inner and outer seal elements 36, 38 are intact,the pressures sensed by the pressure transducers 126, 132 should besteady and even and the signals received by the controller 134 shouldindicate a consistent, or steady state, difference between the twopressures. If either of the inner and outer seal elements fails, thepressure downstream of the flow restrictor 124 should fall and therewill be noticeable change in the pressure difference indicated by thesignals the controller 134 receives from the pressure transducers 126,132. The controller 134 is configured to compare the signals receivedfrom the first and second transducers 126, 132 to determine thecondition of the seal system 32. In the event the comparison shows achange in the relative pressures, the controller 134 may determine thatone of the seal elements has failed and output a signal to cause anindicator to provide an indication the seal system 32 has failed. Theindicator may provide a visual indication, for example a flashing lightor another visible warning such as a message on a screen or the like, oran audible alarm, to indicate that the seal system has failed. Thesignal from the controller 134 may be sent to a computerised controlsystem for the vacuum system of which the vacuum system pipe coupling 20rather than to a dedicated indicator associated with the controller andin some examples, the controller may be integrated into such acomputerised control system.

In the event of a failure of one of the seal elements 36, 38 of the sealsystem 32, the other seal element should continue to function so thatthe integrity of the vacuum coupling system 20 is not immediatelycompromised. This allows time for a repair to be organised and scheduledinto the operation of the vacuum system.

The pressurised gas supplied by the gas module 116 may be an inert gassuch as nitrogen or oxygen free nitrogen. Accordingly, if the outer sealelement 38 fails, all that is released to atmosphere is an inert gas,while if the inner seal element 36 fails, the gas entering the flowpaththrough the vacuum system pipe coupling 20 will not trigger a reactionwith the exhaust gases flowing through the coupling. This can beimportant if the gases flowing through the coupling are liable tospontaneously combust.

If the inner seal element 36 failed in the absence of the pressurisedgas in the space 42, the outer seal element 38 would potentially beexposed to the exhaust gases flowing through the vacuum system pipecoupling 20. In many operating environments, this would expose the outerseal element 38 to chemical damage it is less well equipped to withstandthan the inner seal element. However, the pressurised gas supplied intothe space 42 via the first flow path 40 may act as a shield for theouter seal element 38, protecting it from the exhaust gases flowingthrough the vacuum system pipe coupling 20 and thereby prolonging theperiod in which the vacuum system pipe coupling 20 can continue in useprior to being disassembled to allow replacement of the seal system 32.

FIG. 6 shows a sealing system 332 that may be used in the vacuum systempipe coupling 20 in place of the sealing system 32. The sealing system332 comprises an inner seal element 336, an outer seal element 338 and aspacer 390.

The inner seal element 336 is an O-ring made of a material havingrelatively good resistance to chemical attack and good high temperaturestability. The inner seal element 336 may, for example, be made from aperflouroelastomer such as Kalrez® or Perlast®. The inner seal element336 may comprise a core, or substrate, made of a relatively cheapermaterial, such as Viton® coated, sleeved or otherwise suitably coveredwith a relatively expensive outer layer made of Kalrez, Perlast or thelike. The outer seal element 338 is an O-ring made of a material havingrelatively good mechanical properties such as resilience and resistanceto compression set and may, for example, be a fluoroelastomer such asViton®.

In the sealing system 332, the inner and outer seal elements 336, 338are separate bodies and instead of being connected by an integral web asin the sealing system 32, they are separated, or spaced apart, by aseparate spacer 390. The spacer 390 is an annular body having arectangular cross-section and may be made of any suitable metal, forexample aluminum or stainless steel, or an engineering plasticsmaterial. The respective opposed faces 400, 402 of the spacer 390 thatcontact the inner and outer seal elements 336, 338 are slightly concaveto facilitate mating with the seal elements and centre the spacer withrespect to the seal elements.

The spacer 390 has a width W in a direction parallel to the longitudinalaxis of the vacuum system pipe coupling 20 that is less than thediameter (width) of the inner seal element 336 and less than thediameter (width) of the outer seal element 338 an that when the sealelements are disposed between and in engagement with the first andsecond flanges 26, 30, the space 42 is defined between the opposedsealing faces 50, 74 and the inner and outer seal elements 336, 338. Incross-section, the spacer 390 has a longitudinal axis disposedperpendicular to the respective longitudinal (circumferentiallyextending) axes 392, 394 of the inner and outer seal elements 336, 338so that the web is disposed substantially centrally with respect to theseal elements and consequently with respect to the sealing faces 50, 74so as to lie centrally in the space 42.

The spacer 390 is provided with a plurality of transverse through-holes396 to permit pressurised gas supplied from the flow passage 40 to fillthe space 42 on either side of the spacer. At least on one side of thespacer 390, a depression 398 is provided around each through-hole 396.

The sealing system 332 further comprises an inner seal carrier 406 andan outer seal carrier 408. These components may be the same as the innerand outer seal carriers 106, 108 shown in FIGS. 2 to 4 and so will notbe described in detail again.

In the illustrated examples, the securing system comprises a clamp usedto releasably clamp the first and second couplings to one another. It iscurrently envisaged that the securing system will comprise aquick-release clamp the same as, or operating on similar principles toknown quick release clamps used for securing components of KF and NWfittings known in the art. It is to be understood that the invention isnot limited to this and in principle, the securing system may compriseany apparatus or arrangement of components suitable for releasablysecuring the first and second coupling members to one another,including, but not limited to, a plurality of bolts, screws or studsengaging nuts or threaded apertures provided in one of the couplingmembers.

In the illustrated examples, a separate spacer or integral web isprovided to determine the spacing between the inner and outer sealelements. In another example, the spacer may be integral with one of theinner and outer seal elements and separate of the other.

In the example illustrated by FIG. 5, the piping 130 is shown optionallyconnected to the first flow passage in the first coupling member of asecond vacuum system pipe coupling 20. Connecting the pressurised gassupply to the flow passages of a series of vacuum system couplings inthis way allows one gas supply to be used for seal system monitoring intwo of more vacuum system pipe couplings.

In some examples the vacuum system pipe coupling may be marketed as akit, for example to be fitted in existing installations. In otherexamples, the vacuum system pipe coupling may be marketed as a part of avacuum system with the first and second coupling members ready fitted topiping of the vacuum system.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are described asexample forms of implementing the claims.

1. A vacuum system pipe coupling comprising: a first coupling member; a second coupling member; a seal system to be positioned between said first and second coupling members to seal between said coupling members; and a securing system to releasably secure said first coupling member to said second coupling member, said seal system comprising an inner seal element and an outer seal element and said first coupling member being provided with a flow passage configured to conduct a pressurised gas to a space that in use is provided between said inner and outer seal elements to shield said outer seal element from fluid flowing through said pipe coupling in the event of failure of said inner seal element.
 2. The vacuum system pipe coupling as claimed in claim 1, wherein said inner and outer seal elements are separated by a spacer that is configured to be received in said space.
 3. The vacuum system pipe coupling as claimed in claim 2, wherein said spacer is integral with at least one of said inner and outer seal elements.
 4. The vacuum system pipe coupling as claimed in claim 2, wherein said inner seal element has a width, said outer seal element has a width and said spacer comprises at least one portion having a width less than the respective said widths of said inner and outer seal elements.
 5. The vacuum system pipe coupling as claimed in claim 2, wherein said second coupling member is provided with a flow passage and said spacer is provided with at least one through-hole configured to permit pressurised gas from said flow passage of said first coupling member to flow to said flow passage of said second coupling member.
 6. The vacuum system pipe coupling as claimed in claim 5, comprising a first pipe to be connected to an inlet end of said flow passage of said first coupling member, a flow restrictor and a first transducer to be connected with said first pipe so that said first transducer can provide signals indicative of the pressure of a pressurised gas flowing in said first pipe pressure upstream of said flow restrictor, a second pipe to be connected with an outlet end of said flow passage of said second coupling member, a second transducer to be connected with said second pipe to provide signals indicative of the pressure of said pressurised gas flowing in said second pipe and a determining unit that receives said signals and determines seal system condition based on a comparison of the respective said seals received from said first and second transducers.
 7. The vacuum system pipe coupling as claimed in claim 1, wherein said outer seal element comprises a first material and said inner seal element comprises a second material that is different to said first material.
 8. The vacuum system pipe coupling as claimed in claim 7, wherein said first material has a better compression set characteristic than said second material.
 9. The vacuum system pipe coupling as claimed in claim 7, wherein said second material has at least one of better: i) high temperature stability than said first material; and ii) resistance to chemical corrosion than said first material.
 10. The vacuum system pipe coupling as claimed in claim 7, wherein said first material is a fluoroelastomer.
 11. The vacuum system pipe coupling as claimed in claim 7, wherein said second material is a perflouroelastomer.
 12. The vacuum system pipe coupling as claimed in claim 7, wherein said inner seal element comprises a substrate and said second material is carried on said substrate.
 13. The vacuum system pipe coupling as claimed in claim 7, wherein at least one of said first and second materials has a Shore hardness of 60 to
 80. 14. The vacuum system pipe coupling as claimed in claim 1, wherein said first coupling member comprises a first flange having a first sealing surface, said second coupling member comprises a second flange having a second sealing surface and in use said sealing system is disposed between and in engagement with said first and second sealing surfaces, said space being defined between said first and second sealing surfaces and said inner and outer seal elements.
 15. A vacuum system comprising: a first pipe; a second pipe; and a vacuum system pipe coupling said first pipe to said second pipe, wherein said vacuum system pipe coupling comprises: a first coupling member connected with said first pipe; a second coupling member connected with said second pipe; a seal system positioned between said first and second coupling members and sealing between said coupling members; and a securing system releasably securing said first coupling member to said second coupling member, said seal system comprising an inner seal element and an outer seal element spaced from said inner seal element and said first coupling member being provided with a flow passage configured to conduct a pressurised gas to a space between said inner and outer seal elements to shield said outer seal element from fluid flowing through said pipe coupling in the event of failure of said inner seal element.
 16. The vacuum system as claimed in claim 15, wherein said inner and outer seal elements are separated by a spacer that is received in said space.
 17. The vacuum system pipe coupling as claimed in claim 16, wherein said spacer is integral with at least one of said inner and outer seal elements.
 18. The vacuum system as claimed in claim 16, wherein said inner seal element has a width, said outer seal element has a width and said spacer comprises at least one portion having a width less than the respective said widths of said inner and outer seal elements.
 19. The vacuum system as claimed in claim 16, wherein said second coupling member is provided with a flow passage and said spacer is provided with at least one through-hole configured to permit pressurised gas from said flow passage of said first coupling member to flow to said flow passage of said second coupling member.
 20. The vacuum system as claimed in claim 19, further comprising a pressurised gas supply connected with said flow passage of said first coupling member by first piping, a flow restrictor connected with said first piping and a first transducer to provide signals indicative of pressurised gas pressure upstream in said first piping of said flow restrictor, second piping connected with said flow passage of said second coupling member to conduct said pressurised gas away from said vacuum system pipe coupling, a second transducer connected with said second piping to provide signals indicative of pressurised gas pressure in said second piping and a determining unit that receives said signals from said first and second transducers and judge seal system condition based on a comparison of the respective said signals received from said first and second transducers.
 21. The vacuum system as claimed in claim 15, wherein said outer seal element comprises a first material and said inner seal element comprises a second material that is different to said first material.
 22. The vacuum system as claimed in claim 21, wherein said first material has a better compression set characteristic than said second material.
 23. The vacuum system as claimed in claim 21, wherein said second material has at least one of better: i) high temperature stability than said first material; and ii) resistance to chemical corrosion than said first material.
 24. The vacuum system as claimed in claim 21, wherein said first material is a fluoroelastomer.
 25. The vacuum system as claimed in claim 21, wherein said second material is a perflouroelastomer.
 26. The vacuum system as claimed in claim 21, wherein said inner seal element comprises a substrate and said second material is carried on said substrate.
 27. The vacuum system as claimed in claim 21, wherein at least one of said first and second materials has a Shore hardness of 60 to
 80. 28. The vacuum system as claimed in claim 15, wherein said first and second pipes receive exhaust gases from a vacuum pump.
 29. A method of protecting a seal system of a vacuum system pipe coupling comprising a first coupling member releasably secured to a second coupling member by a securing system with said seal system sealing between said first and second coupling members, wherein said sealing system comprises an inner seal element and an outer seal element spaced from said inner seal element and said method comprises flowing a pressurised gas through a flow passage provided in said first coupling member to a space defined between said inner and outer seal elements so that said pressurised gas can shield said outer seal element from gases flowing through said vacuum system pipe coupling in the event said inner seal element fails.
 30. The method as claimed in claim 29, wherein said pressurised gas is an inert gas.
 31. The method as claimed in claim 29, wherein said gases flowing through said vacuum system pipe coupling are exhaust gases from a vacuum pump.
 32. The method as claimed in claim 29, wherein said gases flowing through said vacuum system pipe coupling are at a first pressure and said pressurised gas is at a second pressure greater than said first pressure.
 33. The method as claimed in claim 32, wherein both said pressures are above atmospheric pressure.
 34. A vacuum system pipe coupling comprising: a first coupling member; a second coupling member; a seal system to be positioned between said first and second coupling members to seal between said coupling members; and a securing system to releasably secure said first coupling member to said second coupling member, said seal system comprising an outer seal comprising a first material and an inner seal comprising a second material different to said first material.
 35. The vacuum system pipe coupling as claimed in claim 34, wherein said inner and outer seal elements are separated by a spacer that is configured to be received in said space.
 36. The vacuum system pipe coupling as claimed in claim 35, wherein said spacer is integral with at least one of said inner and outer seal elements.
 37. The vacuum system pipe coupling as claimed in claim 35, wherein said inner seal element has a width, said outer seal element has a width and said spacer comprises at least one portion having a width less than the respective said widths of said inner and outer seal elements.
 38. The vacuum system pipe coupling as claimed in claim 34, wherein said first material has a better compression set characteristic than said second material.
 39. The vacuum system pipe coupling as claimed in claim 34, wherein said second material has at least one of better: i) high temperature stability than said first material; and ii) resistance to chemical corrosion than said first material.
 40. The vacuum system pipe coupling as claimed in claim 34, wherein said first material is a fluoroelastomer.
 41. The vacuum system coupling as claimed in claim 34, wherein said second material is a perflouroelastomer.
 42. The vacuum system pipe coupling as claimed in claim 34, wherein said inner seal element comprises a substrate and said second material is carried on said substrate.
 43. The vacuum system pipe coupling as claimed in claim 34, wherein at least one of said first and second materials has a Shore hardness of 60 to
 80. 